Seismic surveys are often used by natural resource exploration companies and other entities to create images of subsurface geologic structure. These images are used to determine the optimum places to drill for oil and gas and to plan and monitor enhanced resource recovery programs among other applications. Seismic surveys may also be used in a variety of contexts outside of natural resource exploration such as, for example, locating subterranean water and planning road construction.
A seismic survey is normally conducted by placing an array of vibration sensors (accelerometers or velocity sensors sometimes called “geophones”) on the ground, typically in a line or in a grid of rectangular or other geometry. Vibrations are created by an energy source such as, for example, explosives or a mechanical device such as a vibrating energy source or a weight drop. The creation of vibrations by the vibration source may be referred to as a source event. Multiple source events may be used for some surveys. The vibrations from the source events propagate through the earth, taking various paths, refracting and reflecting from geological features such as discontinuities in the subsurface, and are detected by the array of vibration sensors. Signals from the sensors are amplified and digitized, either by separate electronics or internally in the case of “digital” sensors. In some cases, such as in populated areas, passive systems may be employed. In passive systems, rather than using a source to generate seismic events, the array may opportunistically utilize seismic events occurring naturally or generated by events outside the control of the survey operator.
The digital data from the sensors of the array is eventually recorded on storage media, for example magnetic tape, or magnetic or optical disks, or other memory device, along with related information pertaining to the survey. The survey may include multiple source events and/or the active sensors that may move such that the process is continued until multiple seismic records is obtained for a number of source events to comprise a seismic survey. Data from the survey are processed on computers to create the desired information about subsurface geologic structure. In this regard, the seismic information from the sensors of the array is generally synchronized and combined to generate image information that can be interpreted to yield the desired survey result. In general, as more sensors are used, placed closer together, and/or cover a wider area, the quality of the resulting image will improve. It has become common to use thousands of sensors in a seismic survey stretching over an area measured in square kilometers.
Several modes have been developed for reading out the data from the seismic units (e.g., conventional geophones or other units of a seismic survey). Conventionally, individual seismic units are connected by cables to form a line. Multiple lines are then generally distributed across the survey area, often interconnected by a backhaul line or “backbone.” When such systems are practical and functioning properly, they provide substantial bandwidth for quickly reading out large volumes of data. However, in many cases, hundreds of kilometers of cables have been laid on the ground and used to connect the seismic units of such arrays. Large numbers of workers, motor vehicles, and helicopters are often used to deploy and retrieve these cables and the associated seismic sensors. Exploration companies would generally prefer to conduct surveys with more sensors located closer together. However, additional sensors require even more cables and further raise the cost of the survey. Economic tradeoffs between the cost of the survey and the number of sensors generally demand compromises in the quality of the survey.
In addition to the logistic costs, cables connecting sensors create reliability problems. Besides normal wear-and-tear from handling, they are often damaged by animals, vehicles, lightning strikes, and other problems. Considerable field time is expended troubleshooting cable problems. The extra logistics effort also adds to the environmental impact of the survey, which, among other things, adds to the cost of a survey or eliminates surveys in some environmentally sensitive areas.
To avoid some of these difficulties, cableless readout modes have been developed. These include nodal and wireless readout systems. In nodal systems, seismic units are deployed in arrays, typically in similar configurations to conventional cabled arrays. However, instead of reading out seismic data via cables lines, the data is generally stored at each unit until the conclusion of the survey. The data can then be read out on a unit-by-unit basis, for example, by retrieving the units or removable memory, or by porting each unit to a portable data collection unit either via a physical connector or via near field communications.
In wireless readout systems, data is generally read out from individual seismic units while the survey is ongoing, via wireless communications. That is, a unit can be read out from its position in the array to a central collection point without requiring a worker to visit the unit. This may occur in substantially real-time (e.g., as data is being acquired) or on another basis. While there is some latency associated with reading out data from these systems in real-time operation, e.g., associated with serial data transfer, these systems are often referred to as real-time systems to distinguish them from blind systems that generally do not involve reading out data with the survey is ongoing. Such wireless communications may be transmitted serially from unit-to-unit en route to a central collection point, or individual units may communicate directly with a base station. These various types of systems have generally operated in separate but occasionally competing spheres.